1. Field of the Invention
The present invention relates to a sealed rechargeable battery having a volume-efficient closure assembly configuration with a safety feature which operates reliably when battery internal pressure reaches a predetermined level.
2. Description of the Related Art
Sealed rechargeable batteries are small and lightweight and have high energy density, and are therefore used for various applications ranging from consumer equipment such as mobile phones to driving power sources of electric vehicles or electric tools. Lithium ion rechargeable batteries, in particular, which are used in present day consumer equipment, generally have the following closure assembly design.
In one design shown, for example, in Japanese Patent Publication No. 11-339767, a metallic filter with an aperture, which serves as an internal terminal of the battery, accommodates a metallic foil with a thin portion, a resin inner gasket, a PTC (Positive Temperature Coefficient) element with an opening, a metallic safety vent with a thin portion and a downwardly protruding portion, and a metallic cap that serves as an external terminal, placed in this order. The peripheral edge of the metallic filter is crimped to provide a seal. The downwardly protruding portion of the metallic safety vent is welded to the metallic foil with the gasket interposed therebetween. When the battery internal pressure increases abnormally by accidental overcharging, the rising pressure is applied to the metallic foil first, whereby the protruding portion of the metallic safety vent is pushed up and inverted. As the pressure increases further, part of the thin portion of the metallic foil breaks open. The pressure is applied through this opening to the metallic safety vent, ripping apart the foil from the weld joint and thereby thoroughly breaking the foil, so that the current path is interrupted and generation of gas inside the battery is suppressed. Another safety feature of the battery is that the metallic safety vent breaks to release gas to the outside in case of abnormal increasing of internal pressure due to some fault.
For electric vehicle or electric tool applications, however, the PTC element that has high resistance cannot be used in the configuration shown in Japanese Patent Publication No. 11-339767 because the battery is discharged at a high rate even in normal use. In the configuration shown in the above-mentioned patent document, however, the rupture pressure of the thin portion of the metallic foil and metallic safety vent is controlled by the aperture area of the PTC element that is disposed above, and therefore a spacer having an aperture is required as a substitute for the PTC element. Moreover, since the battery internal pressure tends to rise as compared to consumer equipment applications because of heat inside due to the high-rate discharge, the rupture pressure of the metallic foil needs to be set high so that the current path is not interrupted accidentally during normal use, but is interrupted in an abnormal state caused by overcharging or the like.
To make the rupture pressure higher, for example, the thin portion may be made thicker. On the other hand, this will increase deformation of the metallic foil until its thin portion is pulled apart. As the inversion distance of the protruding portion of the metallic safety vent and the deformation amount of the metallic foil are controlled by the aperture area of the spacer, if, for example, the inversion distance is too small and the deformation amount of the foil is too large for a given rupture pressure, then the thin portion may not thoroughly break apart and may remain partly connected, in which case the current path is not interrupted even when the pressure reaches a predetermined level in an abnormal state.
On the other hand, if the aperture area of the spacer is increased so as to provide a sufficient inversion distance for the protruding portion of the metallic safety vent, the metallic foil is allowed to deform sufficiently and to break apart thoroughly owing to the large inversion distance of the protruding portion of the safety vent. However, a sufficient inversion distance means large deformation of the metallic safety vent, for which the metallic cap requires to have a higher top, which leads to an increase in the volume of the closure assembly.